SLE is a chronic autoimmune disease whose origin is still poorly understood, for which treatment is often only moderately successful or carries severe side effects. Rapamycin, an FDA-approved drug, was effective in limiting disease activity in a cohort of SLE patients, and in that cohort specifically attenuated molecular features characteristic of T cells from SLE patients related to calcium homeostasis. The potential for rapid clinical application of rapamycin holds great promise for treatment of SLE patients. The proposed research is directed towards understanding rapamycin's effect in SLE, both to better understand how rapamycin's target pathway is important for SLE pathogenesis and to better guide the design of new therapies. Rapamycin achieves its effects by blocking the signaling pathway controlled by the mammalian target of rapamycin (mTOR), simultaneously binding the protein FKBP12, known to affect calcium channels. mTOR is known to respond to mitochondrial potential, which is elevated in SLE T cells. Since rapamycin is effective in SLE and has specific effects on molecular features of the disease, it is likely that the mTOR pathway is overactive in SLE T cells, potentially due to increased mitochondrial potential. In order to investigate the mechanisms underlying rapamycin's effect in SLE, we have outlined specific aims: Specific Aim #1: We will determine the role of mitochondrial hyperpolarization and generation of reactive oxygen species (ROS) in up-regulation of the mTOR pathway in primary T cells. Specific Aim #2: We will assess the involvement of the mTOR pathway in T cells isolated from SLE patients versus those isolated from healthy controls, and test the involvement of nitric oxide and ROS. Specific Aim #3: We will investigate the role of FKBP12 in mediating the rapamycin-sensitive dysregulation of calcium homeostasis in SLE T cells. Systemic lupus erythematosus (SLE) is a disease that afflicts between approximately 150,000 Americans, limiting their quality of life and predisposing them to a variety of potentially lethal health problems such as heart attacks and kidney failure. The existing treatment options are often inadequate to manage the disease and can cause serious side effects, though we have shown that rapamycin, a drug already approved by the FDA for other purposes, can be successful in controlling SLE and is well-tolerated by patients. The research we propose here is guided at understanding how the drug has its effects in SLE patients, and will help us to better understand how the disease works.